Hemophilia A is an X-linked recessive genetic bleeding disorder caused by a deficiency or functional defect in coagulation factor VIII (FVIII). There is currently no cure for hemophilia A and patients receive infusion of FVIII concentrates or recombinant proteins at the time of bleeding. Although this treatment regimen has increased the life expectancy of hemophiliacs significantly, it is inconvenient and has potentially serious complications such as the development of inhibitory antibodies to FVIII, which occurs in approximately 25% of patients, rendering them refractory to further treatment. The objective of this research is to evaluate the curative efficacy of retroviral vectors encoding modified human FVIII transgenes targeted to hematopoietic stem cells (HSCs) in a murine hemophilia A model. HSCs are an attractive target cell population for hemophilia A gene therapy because they are readily accessible for ex vivo genetic modification and allow for the possibility of sustained expression of a FVIII transgene in circulating peripheral blood cells for the recipient's lifetime. Moreover, a potential benefit of targeting HSCs is the possibility of inducing immunological tolerance to the FVIII transgene product. For almost two decades, our laboratory has been designing and optimizing retroviral vectors for gene transfer studies of HSC biology and gene therapy modeling. In particular, our MSCV (murine stem cell virus) retroviral vector is in use in several HSC gene therapy trials currently underway in the United States. However, the emergence of adverse events in a French clinical trial for X- linked severe combined immunodeficiency disease demands a reevaluation of the risks of retroviral-induced mutagenesis. Therefore, building upon our recent success at achieving clinically-relevant FVIII plasma levels in hemophilia A mice by MSCV-based HSC-directed gene delivery, our Specific Aims are: (1) To further optimize FVIII transgene sequences for more efficient secretion in hematopoietic cells and decreased immunogenicity of the protein;(2) To develop nonmyeloablative HSC transplant conditioning regimens that allow sufficient levels of transgene molecular chimerism for long-term therapeutic FVIII production and tolerance induction;and (3) To create biologically safer FVIII retroviral vectors - devoid of transcriptional regulatory elements within their long terminal repeats and flanked by enhancer/promoter-blocking elements - displaying reduced HSC genotoxicity.